The present invention relates to a high-frequency lamp, which hereinafter is sometimes briefly referred to as an HF lamp. The present invention further relates to a method for operating such a high-frequency lamp.
High-frequency lamps are generally known. The task of each lamp, hence also of a high-frequency lamp, is to emit light as efficiently as possible. Each lamp converts energy into light more or less efficiently. A great amount of dissipated heat often develops during the conversion.
Further tasks of lamps are manifold. The emitted light spectrum is often very crucial for the purpose of use. Just the same, some applications such as vehicle headlights and projectors require lamps having light sources that are as spot-shaped as possible.
The description of the prior art in the following shall be restricted to electric lamps. These may be roughly differentiated into light-emitting diodes and lamps having a glass body. The latter group shall be treated in the following. It is subdivided into incandescent lamps and gas discharge lamps.
Incandescent lamps comprise a filament (e.g. of tungsten) within the glass body, and a protective gas. The filament with a melting point of over 3000° C. is typically heated to 2500° C. According to Planck's radiation law, a light spectrum corresponding to daylight thereby does not thereby result yet for the incandescent lamp, as it rather radiates distinctly more yellow-reddish. Incandescent lamps are operated at a direct or alternate voltage of frequencies up to the kHz range. They do not require any ballast electronics.
Gas discharge lamps, which are in relationship to the present invention, are light sources that use a gas discharge and thereby make use of the spontaneous emission by atomic or molecular-electronic transitions and the recombination radiation of a plasma generated by electric discharges. The gas contained within the bulb of quartz glass (ionization chamber) is often a mixture of metal vapors (e.g. mercury) and noble gases (e.g. argon) and possibly other gases such as halogens. Gas discharge lamps are classified into low-pressure and high-pressure discharge lamps. The former use a glow discharge and the latter an arc discharge. All of these lamps require a ballast. The same contains a starter, which ionizes the gas by means of a voltage pulse in the kV range. Moreover, for permanent operation, the frequency is converted, if necessary, into the kHz range. As a consequence, these lamps are not lamps that are operated in the MHz or GHz range by means of a high-frequency signal.
A special form of the gas discharge lamp is the sulfur lamp. It comprises of a quartz glass globe filled with sulfur and argon as an ionization chamber. Within the glass globe, plasma is generated by way of high-frequency radiation. In contrast to conventional gas discharge lamps, the sulfur lamp does not require any electrodes because of the use of waveguides. Due to the extremely high temperatures developing at the quartz glass of the globe, the same is kept in rotation and thereby cooled. This is caused by a lower column that comprises turbine blade-like fan formations. It turns in the stream of air that is generated within the magnetron (HF energy source of about 1500 W) by a ventilator. In case of the failure of this cooling, the glass globe would melt after 20 seconds.
The luminous efficiency of sulfur lamps is similarly as high as that of energy-saving lamps (fluorescent lamps). They have a balanced light spectrum of a color temperature of approximately 5700 K to 6000 K and, therefore, are very effective white light sources. By regulating the power of the magnetron, sulfur lamps can be dimmed rather well with their color spectrum remaining stable. Due to the high luminous flux, the lamps are in most cases not directly installed in the place of application. Instead, the light is guided into the room by means of optical conductors. This makes this type of lamp easy to maintain.
Due to the relatively high expenditure in terms of devices (power supply for the magnetron, shielding of microwaves, and temperatures), this lamp was commercially not available for a long time. Since 2006, LG Electronics has produced sulfur lamps under the designation “Power Lighting Systems (PLS-lamps, also commercialized as sulfur plasma lamps). They are frequently used as lighting in television studios or as artificial lighting for plants.
From “Emission Properties of Compact Antenna-Excited Super-High Pressure Mercury Microwave Discharge Lamps,” T. Mizojiri, Y. Morimoto, and M. Kando in Japanese Journal of Applied Physics, Vol. 46, No. 6A, 2007, and “Numerical analysis of antenna-excited microwave discharge lamp by finite element method,” M. Kando, T. Fukaya and T. Mizojiri; 28th IC-PIG, Jul. 15-20, 2007, Prague, Czech Republic, high-frequency lamps are known that operate at small high-frequency powers (30-100 W) and comprise, instead of the waveguide coupling, a coupling over a TEM line (coaxial line) including an inner conductor electrode. Since these lamps take advantage of the long wires of a gas discharge lamp as an antenna, these lamps should be referred to in the following, in a more appropriate way, as HF antenna lamps.
These lamps, just as the sulfur lamps, do not comprise an impedance transformer, however. The requirements for the frequency stability of the high-frequency generator hence are low in these lamps.
A disadvantage of these known gas discharge lamps however is that the technology for these lamps requires expenditure and, therefore, is costly. In addition, they are only available as a power lamp of approximately 1500 W. Moreover, all of the gas discharge lamps known to date need a separate circuit for igniting the plasma. For this, voltages in the kV range are necessary. In the hitherto known high-frequency lamps, which can do without a circuit for the ignition, the disadvantage arises, in particular, that they need very much power (over 30 W microwave power). Furthermore, gas discharge lamps act as antennas. In practice, this has the significant disadvantage that high-frequency radiation is emitted to a higher degree. Such lamps are not allowed due to this radiation.
The discharge lamps used as energy-saving lamps cannot be dimmed, a fact that represents a very important disadvantage in practical use.
Since the previous high-frequency lamps do not have impedance transformers in the high-resistance range, very high currents flow through the electrodes. Since these are made of materials such as tungsten and have a poor surface quality, the ohmic losses are very high.
One task of the present invention correspondingly consists in proposing a high-frequency lamp that avoids the above-mentioned disadvantages or at least reduces the effects thereof, in particular in proposing a high-frequency lamp that can be used both as a high-pressure and a low-pressure gas discharge lamp and is especially suited for improving properties such as efficiency, emission spectrum, cost, and durability. A further task of the present invention consists in proposing a method for the operation of such a high-frequency lamp.